The present invention relates to APC (automatic power control) laser diode driver capable of extinction-ratio control.
Laser modules which include a light-emitting circuit having a laser diode (LD) and a light-receiving circuit having a photodiode (PD) are known in the field of optical communication. The LD of the light-emitting circuit produces a predetermined optical output power when a bias current is added to a pulse current which is responsive to input data, and also outputs monitoring light for APC. The PD of the light-receiving circuit receives the monitoring light outputted from the LD and performs light-to-current conversion. Based on the current resulting from the conversion, the magnitude of the bias current and that of the pulse current of the PD are controlled so that constant optical output power and a constant extinction ratio are obtained.
As is well known in the art, temperature variation, process variation, and deterioration caused by an extended period of use, for example, produce variation in the threshold current and conversion efficiency of LDs. In addition, the characteristics of LDs, such as the threshold current, conversion efficiency and variation thereof with temperature, differ completely depending on the manufacture and type of the LD. The LD-to-PD coupling efficiency also varies. In order to obtain optical output power and an extinction ratio which are always constant, the magnitude of the bias current and that of the pulse current have to be initialized appropriately and also has to be optimized at all times according to the conditions of use.
Hereinafter, temperature-variation-induced fluctuation in the threshold current and conversion efficiency of an LD will be described referring to FIGS. 1 through 3.
In FIG. 1, an example of case in which a conventional LD is driven at room temperature (T2), among low temperature (T1), room temperature (T2) and high temperature (T3), is indicated by a solid line representing its current-to-light conversion characteristic (I-P characteristic) 12. In FIG. 1, the character I represents an input current (drive current) into the LD and the character P indicates the optical output power of the LD, while the inclination of the I-P characteristic represents its conversion efficiency. At the room temperature, the threshold current of the LD is Ith2, while a bias current Ib is set equal to the threshold current Ith2 (Ib=Ith2). And a pulse current Ip responsive to input data is superimposed on the bias current Ib. In this case, when the pulse current Ip with a duty ratio of 1 to 1 (high period:low period) is applied to the LD, the LD exhibits a desired high extinction ratio (Pmax/Pmin) such as shown in FIG. 1, and at the same time a maximum optical output power Pmax and a minimum optical output power Pmin show a duty ratio of 1 to 1.
In FIG. 2, an example of case in which the conventional LD is driven at the high temperature (T3) is indicated by a solid line representing its I-P characteristic 13. At the high temperature, the threshold current of the LD changes to Ith3 (>Ith2), while the conversion efficiency thereof becomes lower than at the room temperature. However, if the same bias current Ib (=Ith2) and pulse current Ip as those at the room temperature (T2) are still being applied to the LD, the maximum value of the optical output power P decreases to cause the extinction ratio to deteriorate, as can be seen from the illustrated maximum optical output power Pmax3 and minimum optical output power Pmin3. Further, the duty ratio of the optical output power P deteriorates considerably.
In FIG. 3, an example of case in which the conventional LD is driven at the low temperature (T1) is indicated by a solid line representing its I-P characteristic 11. At the low temperature, the threshold current of the LD changes to Ith1 (>Ith2), while the conversion efficiency thereof becomes higher than at the room temperature. However, if the same bias current Ib (=Ith2) and pulse current Ip as those at the room temperature (T2) are still being applied to the LD, the maximum and minimum values of the optical output power P both rise, as can be seen from the illustrated maximum and minimum optical output powers Pmax1 and Pmin1, thereby also causing deterioration in the extinction ratio.
As described above, the bias current Ib smaller than the threshold current Ith3 as shown in FIG. 2 results in the heavy deterioration in the maximum optical output power, extinction ratio and duty ratio. On the other hand, when the bias current Ib exceeds the threshold current Ith1 as shown in FIG. 3, the extinction ratio deteriorates considerably. In any of these cases, a problem arises in that the communication cannot be performed smoothly, for example.
FIG. 4 illustrates an example of case in which an LD is driven in an ideal manner such that the maximum optical output power, extinction ratio and duty ratio are all kept constant regardless of the ambient temperature. Specifically, at the high temperature (T3), the bias current is raised to Ib3 (=Ith3) in response to the increase in the threshold current from Ith2to Ith3, while the pulse current is increased to Ip3 in accordance with the decrease in the conversion efficiency. At the low temperature (T1), the bias current is reduced to Ib1 (=Ith1) in response to the decrease in the threshold current from Ith2 to Ith1, while the pulse current is decreased to Ip1 in accordance with the increase in the conversion efficiency. These adjustments in the currents allow the maximum and minimum optical output powers Pmax and Pmin as those achieved at the room temperature (T2) to be always obtained irrespective of the ambient temperature.
In order to drive LDs in such an ideal manner, various attempts have been made. Those attempts include a conventional technique in which a beam of monitoring light outputted from an LD is subjected to light-to-electricity conversion performed by a PD, and the resultant electric signal outputted from the PD is inputted into an average-value detection circuit and a peak-value detection circuit. The average-value output voltage and peak-value output voltage outputted from the detection circuits are applied to an operation circuit where a voltage which is proportional to the difference between a voltage which is twice the average-value output voltage and the peak-value output voltage is generated and then fed back to a reference-voltage setting terminal or a bias-current control terminal of a laser drive circuit. At the same time, the average-value output voltage is fed back to a pulse-current control terminal of the laser drive circuit. In this manner, the conventional technique achieves the definite amplitude, upper and lower symmetry and extinction ratio of the optical output power waveform (see Japanese Laid-Open Publication No. 6-164049).
The conventional technique, which permits the average optical output power to be constant, however, has a problem because the difference between the voltage which is twice the average-value output voltage and the peak-value output voltage is controlled to be zero based on the assumption that the minimum optical output power is equal to zero, but the minimum optical output power (≠0) actually exists and produces a corresponding offset which has adverse effect.
In the known technique, in the case of a bias current greater than the threshold current, although the average optical output power is equal to a reference value, there is a possibility that the duty ratio of the optical output power would not be 1 to 1 and that a state of equilibrium would be achieved when the difference between the voltage which is twice the average-value output voltage and the peak-value output voltage is zero. In such a case, the desired maximum optical output power, extinction ratio and duty ratio might not be obtained